Membrane tanks are preferably used as containers for the storage of liquids, especially for the storage of fuels, which are impinged upon and pressurized by a pressure gas that is separated from the respective liquid by a separating membrane in the tank. Such uses typically arise in space travel under conditions of weightlessness, but also under gravity, for example in rapidly or strongly position-variable systems, such as aircraft or submarines. While the outer shape of such tanks is mostly embodied as a sphere, the membrane located in the interior of these tanks is usually a half-spherical shaped body, which is equatorially clamped to the tank along the circumferential rim of the membrane.
The membranes used in such tanks mostly consist of chemically compatible elastomer material. The elastomeric membrane must be held without relaxation, i.e. without loss of its elastic characteristic, in a durable leak-tight manner in the equatorial clamping arrangement of the tank. The main part of the tank volume within the half-spherical volume of the membrane is liquid-filled, while a small part of the tank volume, namely that outside of the membrane, is filled with the pressure gas, whereby the gas pressurizes the entire tank interior by pressing against the membrane and thereby pressurizing the liquid on the liquid side of the membrane.
For a liquid removal, that is to say for emptying the tank, an increased gas pressure is needed for driving out the liquid against the flow resistance imposed by the system as well as against the internal pressure of the downstream system. This purpose is served by a pressure reservoir on the pressure gas side, which can be arranged either within or outside of the tank. The required tank pressure can either be taken out in the relaxation or slackening mode, the so-called “blow-down” mode, can be established and regulated via a constant pressure regulator in the case of an external gas tank.
With increasing removal of liquid, an inversion process of the membrane through its equatorial plane takes place, all the way until the total emptying of the entire tank volume. In contrast to plastic polymer membranes, elastomer membranes withstand such inversion processes without the formation of rips or tears and leaks caused thereby. However, elastomer membrane materials having a sufficient compatibility with the liquids used in space flight are not available, because such liquids are chemically very aggressive, such as the oxidizer types based on nitrogen dioxide. Thus, elastomer membranes are not suitable for membrane tanks for use in space flight systems. Rather, all chemically compatible synthetic material membranes suitable for the oxidizer types used for space flight belong to the material class of the perfluoro polymers. These, however, are of a plastic nature, and as plastic materials are not suitable for a direct clamping-in inside the tank, because they lack an inherent durable stable resilient restoring force.
The mechanical problems arising in the use of plastic polymer materials for tank separating membranes are as follows:                1.) tear formation during the inversion process,        2.) permeation and leaking through pores, especially with thin-walled materials,        3.) creeping under the influence of pressure forces with the result of an attachment that is not durably secure over time, and a merely temporary leak-tightness of the membrane clamping arrangement.        
While the first two problems can be regarded as already having been solved in the known tanks that are being used today, the problem of the creeping of the membrane at the clamping arrangement has previously still not been satisfactorily solved. While elastic membrane materials, due to their elastic resilient restoring forces, can be fixed and sealed by a clamping arrangement in a metallic surrounding, for plastic materials an elastic surrounding for the clamping and sealing function is previously not known. This elastic characteristic of the clamping surrounding must be able to compensate the creeping of a plastic polymer material and therewith a mechanical loosening as well as a possible loss of seal-tightness. For maintaining the clamping and sealing characteristics in the long term, thus for the entire time duration of a mission or some other application, it is decisively important that at the end of the creeping of the polymer, a stable force equilibrium arises between the spring forces of the clamping arrangement and the active resilient restoring forces of the membrane. This must be ensured above all, also under the influence of a chemically aggressive environment and especially in connection with a swelling due to the influence of fuels, with simultaneously arising alternating loads due to positive and negative temperature changes as well as by variations of the tank pressure, which can have an influence on the tension of the clamping arrangement.